Cleaning of exhaust gases by using catalytic converters has nowadays become more and more common even in smaller internal combustion engines of two-stroke type, e.g. in handheld power-driven working tools, such as chain saws and grass trimmers. Demands concerning weight and size of the exhaust cleaning equipment have led to the development of so called catalytic mufflers which are mounted directly to the exhaust port of the cylinder. A typical catalytic muffler has a sealed housing with an inlet as well as an outlet port, and a catalytic element mounted in the housing for cleaning the exhaust gases leaving the muffler. Furthermore it is often advantageous to release the exhaust gases through the lid of the muffler, see WO9602741.
A consequence of the introduction of catalytic conversion is that the temperature of the exhaust gases has increased considerably owing to the reactions in the catalytic converter. The compact design of the above-mentioned type of catalytic muffler results in that the exhaust gases will leave the muffler relatively soon after they have passed through the catalytic converter, and thus extremely hot exhaust gases are released from the muffler. Furthermore it is common that the plate parts of the muffler will enclose the catalytic converter element relatively tight, and so called “hot spots”, i.e. surface areas of the catalytic element getting an extremely high temperature, can spread even to surrounding plate parts. When fresh air, i.e. air rich in oxygen surrounding the engine, comes into contact with the extremely hot exhaust gases, or with these “hot spots”, flames could occur. This could lead to an increased risk of fire, particularly since this type of smaller engines are often used in tools handled in fire risk environments with plenty of inflammable material.
In order to essentially eliminate the risk of fire caused by sparks a catalytic muffler of this kind is often equipped with a spark-arrester screen, which even is a requirement in US-legislation. For obtaining a satisfactory result the spark-arrester screen must be so fine-meshed that it gets a relatively short lifetime. However, such a spark-arrester screen can only to a limited extent put out flames.